The present invention relates to forged rolls for cold rolling and more particularly to working rolls for rolling iron and steels, non-ferrous metals and their alloys at temperatures lower than or equal to 100.degree. C. and possibly to backing rolls used in multi-roll rolling mills.
In order to ensure excellent endurance in service at the lowest cost and working rolls must have a number of characteristics in the state of utilization, namely:
1. A high surface hardness of between 90 and 105 Shore C according to the products to be rolled.
2. Great depth of the hardened layer which will permit limiting, or even eliminating, retreatments which may be necessary for maintaining the desired hardness throughout the given depth of utilization of the roll.
3. High resistance to wear by abrasion.
4. A controlled content of residual austenite of the hardened layer; it being understood that excessively high contents of residual austenite are harmful in that they promote cracking under service stress.
5. A dendritic structure of the surface layers which is sufficiently homogeneous in order to avoid a phenomenon of an extremely fine pitting of the sheet which is given the name of "toad skin" or "orange peel" in the profession.
A large number of these characteristics may be regulated by a judicious choice of the conditions of manufacture of the cold-rolling rolls and more particularly of the heat treating operations: tempering whereby it is possible to adjust the hardness of the body of the roll, conventional hardening method with heating to a temperature&gt;AC3 of the whole of the roll during the austenitization, surface hardening after heating to a temperature&gt;AC3 solely of a relatively thin layer, more or less well adjusted cooling conditions.
However, the choice of the grade remains primordial for optimizing the required characteristics at the lowest cost.
The grades used at the present time for cold-rolling working rolls of water-hardened forged steel comprise 0.8 to 0.9% carbon, 1.8 to 3.0% chromium and other alloy elements and are illustrated by the conventional grade 83 CDV7 which has in fact a sufficiently high content of carbon to obtain the required high levels of hardness, the contents of Cr, Mo, V are sufficient to obtain a correct hardenability and the formation of many carbides ensuring good wear resistance. With conventional heat treatments followed by an energetic water quenching it is thus possible to obtain easily a surface hardness of 103 Shore C, a depth of 15 mm of a hardened layer having a hardness of .gtoreq.85 Shore C on rolls having a roll body surface diameter of 550 to 650 mm.
With a surface hardening after induction heating at the frequency of 50 Hz, similar surface hardnesses are obtained with however a hardened layer of greater depth, namely about 22 mm.
However, in order to take full advantage of the useful depth of the roll body surface, such hardened depths require a minimum of two retreatments.
These retreatments are expensive and many manufacturers have sought to improve the hardenability of the steel so as to obtain hardened layers having a depth of about 30 mm, which then limits the number of retreatments to a single operation.
In order to increase this depth, attention has been directed to more highly alloyed steels having contents of Cr ranging up to 3% and of Mo up to 0.5%. Apart from the fact that these alloy elements are expensive, the increase in their content has the serious drawback of producing an undesirable amount of residual austenite after the martensitic quenching.
Large amounts of residual austenite may be remedied by a treatment subsequent to the quenching consisting in plunging the roll into liquid nitrogen (sub-zero treatment), but these treatments are delicate to carry out and costly.
Lastly, the increase in the content of the alloy elements Cr, Mo, V results in a banded structure and a dendritic structure which impair the surface quality of the rolled products.